Photometric redshifts for active galactic nuclei with LePHARE for the Vera C. Rubin Observatory

Abstract

Active galactic nuclei (AGNs) play a crucial role in galaxy evolution, but they are a minority of extragalactic sources with diverse spectral energy distributions (SEDs), which depend on their means of selection. Upcoming large-scale surveys such as LSST will identify many AGNs, but analysis tools are not optimized for them. The limited number of photometric bands in these surveys impacts the calculation of photometric redshifts for AGNs, which are essential for scientific advancement. We use LePHARE to demonstrate the impact that a limited number of bands and erroneous assumptions have on the determination of the photometric redshifts of AGNs. We conducted tests on six AGN samples selected using X-ray, radio, infrared, variability, color, and spectroscopic criteria in the COSMOS field, using photometry from HSC-CLAUDS, which is closest in depth and wavelength coverage to LSST. We present the LSST pipeline for LePHARE within the redshift assessment infrastructure layers (RAIL), facilitating comparison between SED fitting and machine-learning algorithms. AGNs that appear as point-like sources in optical data are assigned highly unreliable photometric redshifts if they are processed using galaxy templates. Additionally, shallow all-sky surveys (such as eROSITA, WISE, and ZTF) miss many AGNs. As a result, these ``hidden'' AGNs are often misidentified as galaxies in public survey data, leading to incorrect photometric redshifts. We provide the configurations that are suggested for each type of AGN alongside measures of expected performance as a function of redshift, magnitude, and selection. To facilitate studies with a panchromatic view of AGNs, we also released photometric redshifts and posterior distributions for all AGN sources identified in the COSMOS field using the six criteria, based on 28-band photometry.